1. Field of the Invention
This invention relates to an electronic endoscope for displaying a body-cavity image on a TV monitor, the endoscope having a video-scope with an image sensor and a video-processor. In particular, the present invention relates to adjusting the brightness of an object image.
2. Description of the Related Art
In an electronic endoscope, a video-scope includes an optical light guide extended therethrough, which is formed as a bundle of optical fibers. A video-processor includes a light source, such as a halogen lamp. When the video-scope is connected to the video-processor, a proximal end of the optical light guide is optically connected to the light source. Thus, an object to be photographed is illuminated by light radiating from a distal end of the optical light guide, and an object image is formed on the image sensor provided at the distal end of the video-scope. Then, the object image, formed on the image sensor, is converted into analog image-pixel signals by photoelectric conversion.
The analog image-pixel signals are fed to the video-processor and are suitably processed, so that video signals are generated. The video signals are then output to a TV monitor, thus a body-cavity image (for example, a stomach image) is displayed on the monitor.
In general, to maintain proper brightness of the object image displayed on the monitor, a quantity of light radiating from the distal end of the optical light guide should be automatically regulated. In a conventional endoscope, for regulation of the quantity of light, a stop (diaphragm) provided between the light source and the proximal end of the light guide is controlled, such that the brightness of the object image displayed on the monitor is maintained at a constant level.
In this case, luminance signals are generated from one frame worth of analog image-pixel signals, which are successively extracted from the image sensor, and an average luminance value is successively calculated on the basis of the luminance signals. Then, the stop is controlled on the basis of the difference between the average luminance value and a predetermined reference value. The reference value is, for example, a middle value of a range of luminance values (from 0 to 255). The regulation of the quantity of light is successively performed at regular time-intervals.
By regulating the quantity of light automatically, the brightness of the object image formed on the image sensor is maintained at a proper level. Thus, on the monitor, the object image is always displayed with a proper brightness.
However, where halation occurs on the object image displayed on the monitor. Specifically, when the brightness of the object image displayed on the monitor becomes abnormally high and is whitened, the stop occasionally cannot be driven to a determined position in one drive. This problem is due to the fact that the difference between the average luminance value and the reference value is too large. Therefore, the process of the adjusting of the quantity of light takes a long time when halation occurs on the object image. Namely, the adjusting of the quantity of light cannot be rapidly performed when the brightness of the object image changes considerably.
On the other hand, if the stop is driven without any restriction, hunting, in which the stop does not converge to a proper position for a long time, occurs, so that the brightness of the object image cannot be properly adjusted.
Therefore, an object of the present invention is to provide an electronic endoscope that can rapidly adjust brightness of an object image displayed on a monitor by controlling quality of light radiating from the distal end of the video-scope when halation occurs on the object image.
An electronic endoscope of the present invention includes a video-scope, video-processor, a light source, a luminance value calculator, a light-adjuster, and a determiner. The video-scope has an image sensor, provided at a distal end of the video-scope, on which an object image is formed, and a light guide provided throughout the video-scope, to guide light from a proximal end of the video-scope to the distal end of the video-scope. The video-processor, to which the proximal end of the video-scope and a monitor for displaying the object image are respectively connected, processes image-pixel signals corresponding to the object image, successively read from the image sensor. The light source is provided in said video-processor such that light, emitted from the light source, is guided through the light guide and radiates from the distal end of the video-scope. The luminance value calculator successively calculates a luminance value indicating brightness of the object image, on the basis of said image-pixel signals. The light-adjuster adjusts a quantity of light radiating from the distal end of the video-scope on the basis of a difference between the luminance value and a reference value indicating a proper brightness of the object image at regular-time-intervals, such that the brightness of said object image displayed on the monitor is maintained at constant level. Then, the determiner determines whether halation has occurred on the object image displayed on the monitor, on the basis of the luminance value. Further, the light-adjuster includes a light-reducer. The light-reducer reduces the quantity of light radiating from the distal end of the video-scope by a predetermined percentage independent of the difference between the luminance value and the reference value when halation occurs on the object image.